Natural gas, which is the major source of methane, is usually used as a fuel. The amount of methane present in natural gas varies between 40 to 95 volume percents, depending on its source.
Large scale transportation and utilization of natural gas, however, requires sophisticated pipeline systems which are expensive as such and still further their maintenance is time consuming and costly.
Liquefaction of natural gas is proposed as an alternative method for natural gas transportation, but liquefaction, transfer and re-vaporization of the produced liquid is not only complex and energy consuming, but it also requires very expensive safety systems. Therefore, if methane present in the natural gas can be converted to more easily transportable and more valuable products, such as ethylene and ethane, natural gas utilization will become more efficient and more economical.
At present, the naphta cut produced from fractional distillation of crude oil is the major source of ethylene. Due to increased demand for gasoline, supplying naphta to petrochemical complexes is becoming more and more difficult every day. As a result, the usage of natural gas as a new source of ethylene becomes important.
Since 1982, when Keller and Bhasin first introduced the direct conversion of methane to ethylene, a lot of research work has been carried out in this field. The major problem toward commercialization of this process has been its low yield during the ethane and ethylene production. Reasons for that are undesired methane combustion reactions, which in turn convert some of the methane to carbon monoxide and carbon dioxide. Many efforts for improving the process focused on production of catalysts, which can increase the yields of ethane and ethylene.
According to the prior art, studies, for example a study by Lunsford, compare three types of catalysts useful during a process for producing ethylene: (a) catalysts including manganese and tungsten oxides supported on silicon oxide (silica), (b) catalysts based on barium oxide supported on magnesium oxide and (c) catalysts based on strontium oxide supported on lanthanum oxide.
The catalyst comprising manganese and tungsten oxides supported on silica has shown methane conversion rates of up to 37% and C2-hydrocarbon selectivity of up to 65% at feed flow rates of 1320 ml/min per gram of the catalyst. The initial feed gas comprised 45% methane, 15% oxygen, and 30% an inert gas (by volume), and the process was run under atmospheric pressure.
In 1995, Ding-Jum and co-workers studied two types of catalysts: a manganese tungsten based catalyst supported on silica and also manganese tungsten based catalyst supported on magnesium oxide. A maximum performance was observed for the manganese tungsten based catalyst supported on silica. Such maximum conversion rates correspond to a methane conversion of about 20%, and a C2-hydrocarbon selectivity of about 80%. This performance was achieved at temperatures of 800° C., a methane to oxygen ratio of 7.3 and feed flow rate of 383 ml/min per gram catalyst.
A quite recently published prior art study dated 2002 (author: Sheng-Fu-Ji) reports on a manganese tungsten based catalyst supported on silica. It was found that a maximum C2-hydrocarbon yield amounts to about 19.2% (corresponding to methane conversion of 32.7%) with a C2-hydrocarbon selectivity of 58.6% at a temperature of 800° C., a methane to oxygen ratio of 3 and a feed flow rate of 600 ml/min per gram catalyst, carried out under atmospheric pressure.
In 2003 Shuben Li issued a review paper on the performance of manganese oxide based catalysts supported on silica, promoted by various elements. It was suggested to add a second active component (as sodium tungstate) to a manganese based catalyst supported on silica. As such component a transition metal oxide was used and led to an improvement of the catalytic behaviour. It is reported about a C2 hydrocarbon yield of 23.9% and a selectivity of 64.9% for C2-hydrocarbons at a temperature of 800° C. under atmospheric pressure with a feed flow rate of about 600 ml/min per gram catalyst and a methane to oxygen ratio of 3, provided that 40 vol. % of inert gas are present in the feed gas.
One should bear in mind that the processes described in the aforementioned prior art suffer from the drawback of a rather low C2-hydrocarbon selectivity, which results in the inability for the respective process to be commercialized under economically feasible conditions.
U.S. Pat. No. 4,560,821, U.S. Pat. No. 4,523,050 and U.S. Pat. No. 4,554,395 refer to a group of catalysts wherein the major components are reducible oxides selected from the group of Mn, Sn, In, Ge, Pb, Sb, and Bi.
U.S. Pat. No. 4,777,313 discloses a catalyst family with superior performance compared to the performance described in the before mentioned patent documents. In addition to reducible metal oxides the catalysts described also contains Boron (B) and one element selected from alkali metals or alkaline earth metals as promoter.
In U.S. Pat. No. 5,817,904 yet another group of catalysts are described, which are based on manganese oxide supported on silica and promoted by one alkali metal and one non-metal with a definite molar ratio of non-metal to alkaline metal.
U.S. Pat. No. 4,939,310 refers to catalysts containing manganese oxide in combination with an element from the group of Sn, Ti, W, Ta, Si, Ge, Pb, P, As, Sb, B, Ga or an element from the Lanthanide or Actinide families, which catalysts are promoted with halogen salts of one of alkali or alkaline earth metals.
The catalysts containing halogen elements, however, are subject to the drawback of gradual deactivation due to gradual loss of halogen elements, which in turn leads to an unstable performance at long term operation. As a consequence, the processes cannot be subject to industrial application under an economic point of view.
Hence, there is a strong need to provide a catalyst useful in a process for producing C2-hydrocarbons, such as ethane and ethylene, by direct conversion of methane which catalyst and process show superior quality and efficiency under long term conditions with still a high selectivity of C2-hydrocarbons.